Modular friction welding head and associated systems and methods

ABSTRACT

Modular friction welding heads and associated systems and methods are disclosed herein. A friction welding system in accordance with a particular embodiment includes a carrier fixture positioned to carry a workpiece, a head support positioned proximate to the carrier fixture, and a modular friction welding head releasably carried by the head support. At least one of the carrier fixture and the head support can have a guide structure with a constrained motion path positioned to guide relative motion between the modular friction welding head and the carrier fixture. A controller can be operatively coupled to the modular friction welding head and programmed with instructions to control the operation of the friction welding head.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to U.S. Provisional PatentApplication No. 61/153,957, filed on Feb. 19, 2009 and incorporatedherein by reference.

TECHNICAL FIELD

The present disclosure is directed generally to modular friction weldingheads and associated systems and methods.

BACKGROUND

Friction stir welding (FSW) is a technique used to weld metal bygenerating friction at an interface between the welded components,typically with a rotating spindle. This technique has been used in theaerospace industry to weld large components, for example, rocket fueltanks. While conventional friction stir welding techniques have provensuccessful, there is a continual need in the industry to improve theefficiency and reduce the cost associated with purchasing and operatingthese high-value devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially schematic side view of a fuel tank that can beformed using techniques and systems in accordance with one or moreembodiments of the present disclosure.

FIGS. 2A-2B are partially schematic illustrations of a fuel tank domeand associated components that can be welded using techniques inaccordance with embodiments of the disclosure.

FIG. 3 is a schematic block diagram illustrating a process for weldingcomponents using a modular welding head in accordance with embodimentsof the disclosure.

FIGS. 4A and 4B illustrate a technique for welding components of a fueltank dome in accordance with an embodiment of the disclosure.

FIGS. 5A and 5B illustrate a technique for welding a cylindrical portionof a fuel tank in accordance with an embodiment of the disclosure.

FIG. 6 is an illustration of a process for joining a fuel tank cylinderand a fuel tank dome using techniques in accordance with an embodimentof the disclosure.

FIG. 7A is a partially schematic side view of a friction welding headand guide structure configured in accordance with an embodiment of thedisclosure.

FIG. 7B is a partially schematic cross-sectional end view of anembodiment of the friction welding head and guide structure, takengenerally along line 7B-7B of FIG. 7A.

DETAILED DESCRIPTION

Aspects of the present disclosure are directed generally to modularfriction welding heads and associated systems and methods. Specificdetails of several embodiments of the disclosure are described belowwith reference to manufacturing a rocket fuel tank to provide a thoroughunderstanding of these embodiments. In other embodiments, thesetechniques can be used to form other devices. Several details describingstructures or processes that are well-known and often associated withfriction welding are not set forth in the following description forpurposes of brevity. Moreover, although the following disclosure setsforth several embodiments of the invention, several other embodimentscan have different configurations or different components than thosedescribed in this section. Accordingly, other embodiments may includeadditional elements and/or may lack one or more of the elementsdescribed below with reference to FIGS. 1-7B.

FIG. 1 is a partially schematic, side view of a product 110 that can beformed using techniques and devices described further below. In aparticular aspect of this embodiment, the product 110 can include a fueltank 111, for example, a fuel tank suitable for liquid-fueled rockets.The fuel tank 111 can include a cylinder or cylindrical portion 113connected to oppositely facing domes 112. The cylinder 113 can be formedfrom a rolled sheet that is welded at a cylinder weld 116. Each of thedomes 112 can be formed from multiple gores 114 (each having a partiallyspherical surface) that are joined to each other at corresponding gorewelds 115. Each of the domes 112 is then attached to the intermediatecylinder 113 at a corresponding dome/cylinder weld 117.

FIG. 2A schematically illustrates portions of the dome 112 shown in FIG.1, including the gores 114 and the associated gore welds 115. In aparticular embodiment, a polar flange 118 (shown in cross-section inFIG. 2B) can be attached to the upwardly facing end of the dome 112, forexample, to support attachments to other structures. These attachmentscan include structural attachments, fuel lines and/or other elements.The polar flange 118 can include a central opening 119 for access intothe dome 112.

Conventional friction welding techniques typically require threedifferent stations to assemble the fuel tank shown in FIGS. 1 and 2A-2B.These stations include a first station at which the individual gores arewelded to form the domes 112, a second station at which the cylinder 113is welded at the cylinder weld 116, and a third, station at which thedomes 112 are attached to the cylinder 113 at the dome/cylinder welds117. Because each of the foregoing components is typically large and hasa high value, each of the foregoing stations typically includes adedicated friction welding head and controller that are specificallydesigned only for the particular task at that station. Accordingly, inorder to form the fuel tank 111 shown in FIG. 1, a manufacturer musttypically purchase and operate three different friction welding devices,each controlled by a corresponding different friction weldingcontroller. While this approach has proven successful in that thesingle-purpose friction welding heads may be less susceptible to failurebecause they are tailored to a particular task, the foregoing approachis also expensive and can consume a large amount of factory space.Accordingly, embodiments of the present disclosure are directed tomodular friction welding heads that can be moved from one station toanother and can accordingly complete a variety of tasks, rather thanjust a single task.

FIG. 3 is a flow diagram illustrating three stations 101 associated withmanufacturing the fuel tank 111 shown in FIG. 1. These stations 101 caninclude a dome assembly station 101 a, a cylinder assembly station 101 band a tank assembly station 101 c. A system 100 in accordance with anembodiment of the disclosure includes a modular friction welding head120 (e.g., a modular friction stir welding head) that may be moved amongthe three stations 101 a, 101 b, 101 c, thus reducing or eliminating theneed for multiple welding heads. In addition, the modular head 120 canbe controlled by a single controller 160, thus reducing or eliminatingthe need for multiple controllers.

In a particular embodiment, the controller 160 is a computer-basedcontroller that includes hardware and software for executingcomputer-based instructions. Accordingly, certain embodiments describedbelow may take the form of computer-executable instructions, includingroutines executed by a programmable computer. Those skilled in therelevant art will appreciate that such embodiments can be practiced oncomputer systems other than those shown and described below. Thecontroller 160 typically includes one or more processors, memories, andinput/output devices, including keyboard(s) and/or display screens ormonitors. The controller 160 can remain stationary while the modularhead 120 moves from one station 101 to the other. In another embodiment,the controller 160 can be portable. In either embodiment, the controller160 can be connected to the modular head 120 with a communication link121, e.g., a flexible communication link. In a particular embodiment,the communication link 121 includes cables, so as to avoidelectromagnetic interference that may be associated with a wirelesslink. However, in other embodiments, the controller 160 can control themodular head 120 with a wireless link, assuming it is suitablynoise-free. In any of these embodiments, the controller 160 can controlboth the welding action and the motion of the modular head 120 at eachstation 101.

The modular head 120 includes elements typically used to create frictionwelds, e.g., a pin tool, a spindle that rotates the pin tool to createthe friction necessary for welding, and one or two shoulders thatcontrol the flow of metal formed during the weld. Further details of arepresentative modular head 120 are described below with reference toFIGS. 7A-7B. The modular head 120 can be configured to operate with asupport structure or tooling that provides support on the backside ofthe piece that is being welded, or the modular head 120 include aself-reacting device that eliminates the need for such a support. Themodular head 120 can include hydraulics or other drivers/actuators thatprovide the forging force needed to produce the friction weld, or theforce can be produced by another device. Suitable devices include anelectrically operated device, for example, a linear voltage displacementtransducer. The modular head 120 can optionally include a laser trackingdevice or another vision system, for example, one or more micro-cameras.The modular head 120 can still further include a pigtail or otherreceiving device to which the communication link 121 described abovewith reference to the controller 160 is attached. In a particularembodiment, the modular head 120 includes custom-made elements and/orassemblies available from any number of friction stir welding devicemanufacturers including ESAB of Stockholm, Sweden, Nova-Tech ofLynnwood, Wash., and MTS of Eden Prairie, Minn.

FIG. 4A is a partially schematic, side view illustration of a portion ofthe system 100 located at the dome assembly station 101 a shown in FIG.3. FIG. 4B is a top view looking down on the system 100 shown in FIG.4A. In one aspect of an embodiment shown in FIGS. 4A and 4B, the system100 includes a first carrier fixture 105 a that supports multiple gores114. For example, the first carrier fixture 105 a can include a rotarytable 102 carrying tooling 103 that in turn supports multiple gores 114.The rotary table 102 can include provisions (e.g., slots) for supportinggores 114 and associated tooling 103 having multiple diameters,thicknesses, or other dimensions. The tooling 103 can be retractableand/or can have other features, e.g., to support multiple functionsperformed at the dome assembly station 101 a. Such functions can includetrimming in addition to welding. The system 100 can further include afirst guide structure 140 a (e.g., a dome track 122 a) that carries themodular head 120 for movement relative to the gores 114, and that issupported by a first head support 131 a having one or more head supportelements 123, e.g., a central support element 123 a and an outer orperipheral support element 123 b. The central support element 123 a cantelescope, e.g., to handle gores 114 of different diameters and/or toprovide support for the polar flange 118 (FIG. 2B). In general, thefirst guide structure 140 a is positioned close to the expected locationof the gores 114 that are to be welded, so as to reduce the extent towhich the modular head 120 is cantilevered relative to the dome track122 a. In operation, the modular head 120 includes a carriage 127 orother suitable device that moves along a first constrained motion path146 a as the modular head 120 welds neighboring gores 114. The firstconstrained motion path 146 a is curved or arcuate in the embodimentshown in FIGS. 4A and 4B. The curvature of the first motion path 146 acan be in a single plane (e.g., the plane of FIG. 4A), or a transverseplane, or both, depending upon the welding operation to be performed. Inany of these embodiments, after an individual weld is completed, therotary table 102 can rotate to align the next interface betweenneighboring gores 114 with the first motion path 146 a of the modularhead 120.

The first guide structure 140 a can include any suitable arrangement forsupporting the motion of the modular head 120. For example, the firstguide structure 140 a can include a rack and pinion arrangement attachedto a sturdy supporting railing or other structure, as described furtherbelow with reference to FIGS. 7A-7B. The rack-and-pinion arrangement caninclude anti-backlash gearing to improve the accuracy with which themodular head 120 is positioned. In other embodiments, the first guidestructure 140 a can include a C-channel, an arrangement of rods, and/oranother device. In any of these embodiments, the modular head 120 caninclude a drive motor or other drive device that moves the modular head120 relative to the associated guide structure. In another embodiment,the drive device can be carried by the first guide structure 140 aitself. For example, the first guide structure 140 a can include amoving toothed belt, chain, or other “tow rope” type arrangement towhich the modular head 120 is connected.

FIG. 5A is a partially schematic, side elevation view of a portion ofthe system 100 located at the cylinder assembly station 101 b shown inFIG. 3. FIG. 5B is a top view of the system 100 shown in FIG. 5A.Referring to FIGS. 5A and 5B, the system 100 can include a second headsupport 131 b that in turn includes a second guide structure 140 b(e.g., a cylinder track 122 b) that extends upwardly from a base supportelement 123 c adjacent to an outer surface of the cylinder 113. Thecylinder track 122 b carries the modular head 120, e.g., the samemodular head 120 as is used at the dome assembly station 101 a (FIG.4A). Accordingly, the modular head 120 can be detached from the firstguide structure 140 a (FIGS. 4A and 4B) and removably attached to thesecond guide structure 140 b. The second guide structure 140 b defines asecond constrained motion path 146 b which is a straight line in theembodiment shown in FIGS. 5A-5B. Accordingly, the modular head 120 canoperate along both a straight line motion path and a curved motion path.In a particular aspect of this embodiment, cylinder assembly station 101b includes a second carrier fixture 105 b that in turn includes assemblytooling 103 positioned at the inner surface of the cylinder 113 to reactforces provided by the modular head 120. Optionally, the assemblytooling 103 can be connected to the second guide structure 140 b abovethe cylinder 113 for enhanced support. The second guide structure 140 bcan have any of the arrangements described above with reference to thefirst guide structure 140 a, and can be positioned close to the cylinder113 to reduce bending moments. The modular head 120 can perform trimmingoperations, in addition to welding operations.

If the cylinder 113 includes multiple sections and requires multiplewelds, it can be indexed either manually or via a turntable generallysimilar to that described above with reference to FIGS. 4A and 4B. Inanother embodiment, the second carrier fixture 105 b can includecylinder supports 104 for the cylinder 113. The supports 104 can includefixed stanchions with rollers at the interface with the cylinder 113. Inyet another embodiment, the second carrier fixture 105 b can include aring-shaped track that allows the cylinder 113 to rotate relative to themodular head 120. In still another embodiment, the base support element123 c can include a circular track that allows the upwardly projectingcylinder track 122 b and the modular head 120 to orbit around thecylinder 113 prior to performing welding or trimming operations atmultiple circumferential locations around the cylinder 113.

FIG. 6 is a partially schematic, top plan view of the system 100illustrating all three stations 101 a, 101 b and 101 c. As shown in FIG.6, the modular head 120 can be moved from the first guide structure 140a (e.g., the dome track 122 a) at the dome assembly station 101 a to thesecond guide structure 140 b (e.g., the cylinder track 122 b) at thecylinder assembly station 101 b. The modular head 120 can then be movedfrom the cylinder track 122 b to a third head support 131 c at the tankassembly station 101 c. The third head support 131 c can be firmlyanchored in place. Accordingly, the modular head 120 can have a fixedposition relative to the cylinder 113 and the domes 112 at the tankassembly station 101 c. The domes 112 and the cylinder 113 can becarried by a third carrier fixture 105 c having one or more rotarysupports 123 d that rotate these components about a longitudinal axis125 while the modular head 120 forms the dome/cylinder welds 117(FIG. 1) at the junctions between the domes 112 and the cylinder 113.The motion of the domes 112 and the cylinder 113 can accordingly bealong a third (curved) constrained motion path 146 c. The third carrierfixture 105 c can translate along the longitudinal axis 125 tosequentially align each dome 112 with the third head support 131 c, orthe third head support 131 c can translate to provide the samealignment. This arrangement can also be used to weld multipleaxially-positioned sections of the cylinder 113 together to produce acylinder that is elongated along the longitudinal axis 125.

FIG. 7A is a partially schematic, cross-sectional illustration of aguide structure 140 and modular friction welding head 120 configured inaccordance with an embodiment of the disclosure. In one aspect of thisembodiment, the guide structure 140 describes a curved motion path 146that is generally parallel to the curved outer surface of the gore 114upon which the modular head 120 operates. Accordingly, the guidestructure 140 can be used at the first station 101 a described above.The system 100 can further include engagement features 141, which, in aparticular embodiment, include one or more racks 142 carried by theguide structure 140, and one or more corresponding pinions 143 carriedby the modular head 120. The engagement features 141 are generallycommon to multiple guide structures to support the modular, portableaspects of the modular head 120. As discussed above, the pinions 143 caninclude anti-backlash pinions that increase the repeatability with whichthe modular head 120 can be located at any point along the motion path146. The modular head 120 can further include one or more retainerwheels 144 or other devices that engage the outside of the guidestructure 140 to help keep the pinions 143 engaged with the racks 142.The modular head 120 can further include a housing 126 that in turncarries a spindle 129 and a friction stir welding probe 130. The spindle129 and probe 130 rotate about a spindle axis 132 to provide a frictionstir weld at the gore 114, in a manner generally known to those ofordinary skill in the relevant art. The motion path 146 can be curvedabout one or more axes, e.g., one or more axes that are transverse tothe spindle axis.

As the modular head 120 travels along the motion path 146 to produce thegore weld 115, it can be prevented from inadvertently traveling off theguide structure 140 by a releasable stop 145. In a particularembodiment, the releasable stop 145 can be disengaged (e.g., by pivotingthe stop 145 as shown in dashed lines in FIG. 7A) to allow the modularhead 120 to be removed from the guide structure 140 and placed onanother guide structure. The modular head 120 can typically weighthousands of pounds, and is accordingly handled by an overhead crane,lift, or other heavy-duty machinery in a typical embodiment. Thisarrangement can be adjusted to be more robust or less robust, dependingupon the size of the modular head 120.

FIG. 7B is a partially schematic, cross-sectional illustration of theguide structure 140 and modular head 120 shown in FIG. 7A. As shown inFIG. 7B, the engagement features 141 can include two racks 142, and twocorresponding pinions 143. The pinions 143 can form a portion of acarriage 127 that is in turn connected to the rest of the head 120. Thehousing 126 of the head 120 can carry multiple actuators 128, includinga travel actuator 128 a (which rotatably drives the pinions 143), aforce actuator 128 b (which provides a normal force to the spindle 129),and a rotation actuator 128 c (which rotates the spindle 129 about thespindle axis 132). The actuators 128 can include any suitable devices,including hydraulically powered devices, electrically powered devices,or pneumatically powered devices.

One aspect of embodiments of the system 100 described above withreference to FIGS. 1-7B is that they can include a single modular head120 and a single controller 160, with each configured to performdifferent friction welding operations at different stations. Oneexpected advantage of this arrangement is that it can reduce the cost ofproducing friction welds by reducing the number of welding heads and/orcontrollers required to form selected structures. For example, in oneembodiment, a manufacturer need purchase and operate only a singlecontroller 160 and a single modular head 120 to produce a fuel tank thatnormally requires three separate welding heads and three associatedcontrollers.

In other embodiments, the system 100 can include multiple modular heads120 and multiple controllers 160. Even in these embodiments, the cost ofthe overall system 100 may be less than the cost of conventionalsystems. For example, the modular heads 120, even if they number morethan one, may be interchangeable with each other and may accordingly bemanufactured as an off-the-shelf item rather than a custom item.Similarly, even if the system 100 includes multiple controllers 160, thecontrollers 160 may be interchangeable, thus reducing the cost of thecontrollers when compared with the cost of a custom manufacturedcontroller.

From the foregoing, it will be appreciated that specific embodiments ofthe disclosure have been described herein for purposes of illustration,but that various modifications may be made without deviating from thedisclosure. For example, while the foregoing embodiments were describedgenerally in the context of a fuel tank, the foregoing techniques andsystems may be used to form structures other than fuel tanks. Particularstructures and methods were described in the context of friction stirwelding, but may also be applicable to other welding techniques,including for example, friction plug welding. Modular heads can beinterchangeable and/or movable among three stations, as shown in FIG. 3,or other numbers of stations (greater or less than three) in otherembodiments.

Certain aspects of the embodiments described above may be combined oreliminated in other embodiments. For example, in many of the embodimentsdescribed above, the product upon which the modular head operates issupported by assembly tooling. In other embodiments, the assemblytooling may be reduced or eliminated, for example, if the modular headcarries its own backing support for the product. Further, whileadvantages associated with certain embodiments have been described inthe context of those embodiments, other embodiments may also exhibitsuch advantages, and not all embodiments need necessarily exhibit suchadvantages to fall in the scope of the present disclosure. Accordingly,the disclosure can include other embodiments not expressly shown ordescribed above.

1. A friction welding system, comprising: a carrier fixture positionedto carry a workpiece; a head support positioned proximate to the carrierfixture; a modular friction welding head releasably carried by the headsupport; a guide structure carried by at least one of the carrierfixture and the head support, the guide structure having a constrainedmotion path positioned to guide relative motion between the modularfriction welding head and the carrier fixture; and a controlleroperatively coupled to the modular friction welding head and programmedwith instructions to control operation of the friction welding head. 2.The system of claim 1 wherein the guide structure is carried by the headsupport and wherein the modular friction welding head is movablerelative to the carrier fixture along the motion path.
 3. The system ofclaim 2 wherein the modular friction welding head includes a travelactuator operatively coupled to the guide structure to move the modularfriction welding head relative to the guide structure.
 4. The system ofclaim 1 wherein the guide structure is a first guide structure and themotion path is a first motion path, and wherein the system furthercomprises a second guide structure having a second motion path, andwherein the modular friction welding head is removably carriable, as aunit, by both the first and second guide structures, and is movablealong the first motion path when carried by the first guide structure,and moveable along the second motion path when carried by the secondguide structure.
 5. The system of claim 1 wherein the modular frictionwelding head includes a modular friction stir welding head.
 6. Thesystem of claim 1 wherein the motion path is generally linear.
 7. Thesystem of claim 1 wherein the motion path is curved.
 8. The system ofclaim 1 wherein the modular friction welding head includes a spindlethat is rotatable about a spindle axis, and a probe projecting from thespindle along the spindle axis, and wherein the motion path is curvedabout an axis that is transverse to the spindle axis.
 9. The system ofclaim 1 wherein the modular friction welding head includes: a spindlethat is rotatable about a spindle axis; a probe projecting from thespindle along the spindle axis; a housing carrying the spindle;engagement features carried by the housing and positioned to releasablyengage with the guide structure; a first actuator coupled to the spindleto rotate the spindle about a spindle axis; a second actuator coupled tothe spindle to apply a force to the spindle generally along the spindleaxis; and a third actuator coupled to the engagement features to movethe engagement features relative to the motion path and move the modularhead along the motion path.
 10. The system of claim 1, furthercomprising a releasable stop carried by the guide structure, thereleasable stop being changeable between a first configuration with thestop positioned to prevent the modular stir friction welding head frommoving off the constrained motion path, and a second configuration withthe stop positioned to allow the modular stir friction welding head tobe removed from the guide structure.
 11. A friction stir welding system,comprising: a first station having a first carrier positioned to carrymultiple gores of a rocket fuel tank; a first guide structure having afirst constrained, arcuate motion path positioned proximate to the firstcarrier; a second station having a second carrier positioned to carry acylindrical portion of a rocket fuel tank; a second guide structurehaving a second constrained, linear motion path positioned proximate tothe second support; a third station having a third carrier positioned tocarry and rotate both the cylindrical portion of a rocket fuel tank anda tank end formed by welding the multiple gores; a fixed head supportpositioned proximate to the third carrier; a modular friction stirwelding head positionable at any of the first, second and third stationsto (a) move along the first motion path at the first station, (b) movealong the second motion path at the second station, and (c) remain in afixed position at the third station, the modular friction stir weldinghead having engagement features positioned to releasably engage with thefirst and second guide structures; and a controller operatively coupledto the modular friction stir welding head and programmed withinstructions for controlling the welding action and motion of themodular friction stir welding head at each of the first, second andthird stations.
 12. The system of claim 11 wherein the first guidestructure includes a first toothed rack and the second guide structureincludes a second toothed rack, and wherein the engagement featuresinclude an anti-backlash pinion releasably engageable with the first andsecond racks.
 13. The system of claim 11 wherein the modular frictionstir welding head is a first modular friction stir welding head carriedby the first guide structure, and wherein the system further comprises:a second modular friction stir welding head carried by the second guidestructure; a third modular friction stir welding head carried by thefixed head support; and wherein the first, second and third modularfriction stir weld heads are interchangeable with each other.
 14. Amethod for welding a product, comprising: performing a first frictionwelding operation on a product at a first station by moving at least oneof a first modular friction welding head and the product along aconstrained motion path relative to the other; moving the product to asecond station; and performing a second friction welding operation onthe product at the second station with the first modular frictionwelding head or a second modular friction welding head that isinterchangeable with the first modular friction welding head.
 15. Themethod of claim 14 wherein performing a first friction welding operationon a product includes performing the first friction welding operation ona rocket fuel tank.
 16. The method of claim 14 wherein performing afirst friction welding operation includes performing a first stirfriction welding operation.
 17. The method of claim 14 wherein moving atleast one of a first modular friction welding head and the productincludes moving the first modular friction welding head along a curvedmotion path relative to the product.
 18. The method of claim 14 whereinmoving at least one of a first modular friction welding head and theproduct includes rotatably engaging an anti-backlash pinion carried bythe first modular friction welding head with a toothed rack positionedproximate to the product.
 19. The method of claim 14 wherein performingthe second friction welding operation includes performing the secondfriction welding operation with a second modular friction welding headthat is interchangeable with the first modular friction welding head.20. The method of claim 14, further comprising moving the first modularfriction welding head from the first station to the second station andperforming the second friction welding operation with the first modularfriction welding head.
 21. The method of claim 20 wherein moving atleast one of the first modular friction welding head and the productalong a constrained motion path includes moving the first modularfriction stir welding head along a first constrained motion path havinga first shape, and wherein performing the second friction weldingoperation on the product at the second station includes moving the firstmodular friction welding head along a second constrained motion pathhaving a second shape different than the first shape.
 22. A method forwelding a product, comprising: performing a first friction weldingoperation on a product at a first station by moving a modular frictionwelding head along a first constrained motion path relative to theproduct; moving the modular friction welding head to a second station;and performing a second friction welding operation with the modularfriction welding head at the second station by moving the modularfriction welding head along a second constrained motion path having ashape different than a corresponding shape of the first constrainedmotion path.
 23. The method of claim 22 wherein performing one of thefirst and second friction welding operations includes forming a weldalong a straight weld line, and wherein performing the other of thefirst and second friction welding operations includes forming a weldalong a curved weld line.
 24. The method of claim 22, further comprisingcontrolling the welding action and motion of the modular frictionwelding head at both the first and second stations with a commoncontroller.
 25. The method of claim 22 wherein performing the firstfriction welding operation includes rotating a spindle and probe of themodular friction welding head about a spindle axis, and rotating themodular friction welding head relative to the product about a motionaxis that is transverse to the spindle axis.
 26. The method of claim 22wherein performing the first and second friction welding operationsincludes performing first and second friction stir welding operations.27. A method for forming a rocket fuel tank, comprising: positioningmultiple gores at a first station; welding neighboring gores together bymoving at least one of (a) a first modular friction stir welding headand (b) the gores, relative to the other along a first constrainedmotion path to form a tank end cap; positioning a cylindrical portion ofthe tank at a second station; welding an axial seam of the cylindricalportion of the tank at the second station by moving at least one of (a)the cylindrical portion and (b) the first friction stir welding head ora second friction stir welding head that is interchangeable with thefirst, relative to the other along a second constrained motion path;positioning the tank end cap adjacent to the welded cylindrical portionof the tank; welding the cylindrical portion of the tank to the tank endcap at a third station by moving at least one of (a) the cylindricalportion and the tank end cap and (b) the first friction stir weldinghead or a third friction stir welding head that is interchangeable withthe first, relative to the other along a third constrained motion path;and using a single controller to control the processes of weldingneighboring gores, welding the axial seam, and welding the portion ofthe tank to the tank end cap.
 28. The method of claim 27 wherein weldingneighboring gores includes welding the gores with a first modularfriction stir welding head, and wherein welding an axial seam includeswelding the axial seam with a second modular friction stir welding headthat is interchangeable with the first, and wherein welding thecylindrical portion of the tank includes welding the cylindrical portionwith a third modular friction stir welding head that is interchangeablewith both the first and the second modular friction stir welding heads.29. The method of claim 27, further comprising moving a single modularfriction stir welding head from the first station to the second stationto weld the axial seam, and from the second station to the third stationto weld the cylindrical portion of the tank to the tank end cap.